This invention relates to a hydraulic control circuit for a boring device, such as a horizontal boring device for installing utility services and the like.
Horizontal boring machines are used more and more frequently to install or replace utility services. The machines include a surface mounted boring unit which has a rotation motor to rotate a drill string having a drill bit at the end thereof and a thrust motor to thrust the drill string forward in the bore hole. The machine can also be used to backream a hole previously bored to enlarge the hole and pull in the new service.
Many horizontal boring machines require the operator to manually and individually adjust the rotation torque and thrust force which together define the amount of power applied to the downhole boring tool. Maintaining the correct balance of power between these two functions is necessary to maximize the efficiency of the unit. Inexperienced operators find it difficult to keep an efficient balance of thrust and rotation. Although necessary for both the boring of the pilot bore and backreaming to enlarge the size of the bore hole, the need for finite control is usually greater when backreaming. Therefore, a need exists for an apparatus to provide for operation of the machine at optimum efficiency even when used by inexperienced operators.
In accordance with one aspect of the present invention, a hydraulic control circuit is provided for use in a horizontal boring machine which rotates a drilling element with hydraulic fluid at a rotation pressure from a first source of hydraulic pressure and thrusts the drilling element in a given direction with fluid at a thrust pressure from a second source of control pressure. The control circuit includes a mixing zone and a diversion valve connected between the second source of hydraulic pressure and a zone of low pressure. A first passage connects the first source of hydraulic pressure and the mixing zone. The first passage has a flow restrictor so that if the pressure from the first source of hydraulic pressure is high for a sufficient amount of time for the pressure in the mixing zone to be increased to a high level, the diversion valve opens to direct a portion of the flow of fluid from the second source of hydraulic pressure to the zone of low pressure to reduce the pressure from the second source of hydraulic pressure.
In accordance with another aspect of the present invention, a second passage having a flow restrictor is connected between the second source of hydraulic pressure and the mixing zone. The first passage has a flow restrictor slightly larger than the restrictor in the second passage so if the pressure from the first source of hydraulic pressure is low, the pressure in the mixing zone will also be lowered. Conversely, if pressure in the first passage is high, the pressure in the mixing zone will also be higher. Pressure in the mixing zone is highest when both the first and second sources of hydraulic pressure are high. When pressure in the mixing zone is high, the diversion valve opens to direct the flow of fluid from the second source of hydraulic pressure to the zone of low pressure to reduce the pressure from the second source of hydraulic pressure.